This invention relates to a fuel assembly and particularly to a fuel assembly suitable for use in boiling water-type nuclear reactors.
In operation of a nuclear reactor for a definite period, it is necessary to run the initial phase of operational period with an excess reactivity corresponding to the deterioration of reactivity due to burning of fissionable material during the operation. To keep the nuclear reactor in a critical state, it is necessary to control the excess reactivity, and the following methods have been so far used often in combination: a method for inserting control rods of neutron-absorbing material into a core, and a method for charging into a core a fuel assembly containing fuel rods whose fuels are incorporated with burnable poisons of large neutron absorption cross-section such as Gd.sub.2 O.sub.3, etc.
Recently, a fuel directed to an increased burnup, that is, a long life fuel, has been under development. To improve a fuel economy in the long life fuel, it is necessary to enhance the enrichment, but an increase in enrichment leads to the following problems. A first problem is an increase in excess reactivity, which must be controlled in the initial phase of burning, due to the increase in enrichment, and in the case of the conventional fuel assembly using gadolinia (Gd.sub.2 O.sub.3)-containing fuel rods, the number of gadolinia-containing fuel rods must be increased. Furthermore, the average energy level of neutron are enhanced, and consequently the excess reactivity control effect per gadolinia-containing fuel rod is reduced. Thus, the number of gadolinia-containing fuel rods in a fuel assembly must be further increased. A second problem due to the increase in enrichment is an increase in the absolute value of void reactivity coefficient, caused by the enhancement of neutron energy level, and the control rod reactivity worth is lowered.
To solve these problems, such a fuel assembly has been proposed (Japanese Patent Application Kokai (Laid-open) No. 53-41696, FIG. 3), where rods are arranged in 8 rows and 8 lines (8.times.8) in a fuel assembly, and consist of first fuel rods filled with fuel pellets of nuclear fuel material, second fuel rods whose fuel pellets are incorporated with gadolinia, and burnable poison containing moderator rods filled with a moderator material (for example, B.sup.11, beryllium carbon and their compounds, light water and heavy water) and a burnable poison (for example, B.sup.10, Gd and their compounds), as arranged in a channel box. The second fuel rods are arranged one each at four corners of the channel box, totalling to 4. Four burnable poison-containing moderator rods are arranged adjacently to one another at the center of the channel box. The fuel assembly thus has three kinds of rods filled with different materials at the inside, and consequently a process for producing a fuel assembly becomes complicated.